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diff --git a/tools/testing/selftests/bpf/README.rst b/tools/testing/selftests/bpf/README.rst new file mode 100644 index 000000000..d3c6b3da0 --- /dev/null +++ b/tools/testing/selftests/bpf/README.rst @@ -0,0 +1,268 @@ +================== +BPF Selftest Notes +================== +General instructions on running selftests can be found in +`Documentation/bpf/bpf_devel_QA.rst`__. + +__ /Documentation/bpf/bpf_devel_QA.rst#q-how-to-run-bpf-selftests + +========================= +Running Selftests in a VM +========================= + +It's now possible to run the selftests using ``tools/testing/selftests/bpf/vmtest.sh``. +The script tries to ensure that the tests are run with the same environment as they +would be run post-submit in the CI used by the Maintainers. + +This script downloads a suitable Kconfig and VM userspace image from the system used by +the CI. It builds the kernel (without overwriting your existing Kconfig), recompiles the +bpf selftests, runs them (by default ``tools/testing/selftests/bpf/test_progs``) and +saves the resulting output (by default in ``~/.bpf_selftests``). + +Script dependencies: +- clang (preferably built from sources, https://github.com/llvm/llvm-project); +- pahole (preferably built from sources, https://git.kernel.org/pub/scm/devel/pahole/pahole.git/); +- qemu; +- docutils (for ``rst2man``); +- libcap-devel. + +For more information on about using the script, run: + +.. code-block:: console + + $ tools/testing/selftests/bpf/vmtest.sh -h + +In case of linker errors when running selftests, try using static linking: + +.. code-block:: console + + $ LDLIBS=-static vmtest.sh + +.. note:: Some distros may not support static linking. + +.. note:: The script uses pahole and clang based on host environment setting. + If you want to change pahole and llvm, you can change `PATH` environment + variable in the beginning of script. + +.. note:: The script currently only supports x86_64 and s390x architectures. + +Additional information about selftest failures are +documented here. + +profiler[23] test failures with clang/llvm <12.0.0 +================================================== + +With clang/llvm <12.0.0, the profiler[23] test may fail. +The symptom looks like + +.. code-block:: c + + // r9 is a pointer to map_value + // r7 is a scalar + 17: bf 96 00 00 00 00 00 00 r6 = r9 + 18: 0f 76 00 00 00 00 00 00 r6 += r7 + math between map_value pointer and register with unbounded min value is not allowed + + // the instructions below will not be seen in the verifier log + 19: a5 07 01 00 01 01 00 00 if r7 < 257 goto +1 + 20: bf 96 00 00 00 00 00 00 r6 = r9 + // r6 is used here + +The verifier will reject such code with above error. +At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and +the insn 20 undoes map_value addition. It is currently impossible for the +verifier to understand such speculative pointer arithmetic. +Hence `this patch`__ addresses it on the compiler side. It was committed on llvm 12. + +__ https://reviews.llvm.org/D85570 + +The corresponding C code + +.. code-block:: c + + for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) { + filepart_length = bpf_probe_read_str(payload, ...); + if (filepart_length <= MAX_PATH) { + barrier_var(filepart_length); // workaround + payload += filepart_length; + } + } + +bpf_iter test failures with clang/llvm 10.0.0 +============================================= + +With clang/llvm 10.0.0, the following two bpf_iter tests failed: + * ``bpf_iter/ipv6_route`` + * ``bpf_iter/netlink`` + +The symptom for ``bpf_iter/ipv6_route`` looks like + +.. code-block:: c + + 2: (79) r8 = *(u64 *)(r1 +8) + ... + 14: (bf) r2 = r8 + 15: (0f) r2 += r1 + ; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen); + 16: (7b) *(u64 *)(r8 +64) = r2 + only read is supported + +The symptom for ``bpf_iter/netlink`` looks like + +.. code-block:: c + + ; struct netlink_sock *nlk = ctx->sk; + 2: (79) r7 = *(u64 *)(r1 +8) + ... + 15: (bf) r2 = r7 + 16: (0f) r2 += r1 + ; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol); + 17: (7b) *(u64 *)(r7 +0) = r2 + only read is supported + +This is due to a llvm BPF backend bug. `The fix`__ +has been pushed to llvm 10.x release branch and will be +available in 10.0.1. The patch is available in llvm 11.0.0 trunk. + +__ https://reviews.llvm.org/D78466 + +bpf_verif_scale/loop6.bpf.o test failure with Clang 12 +====================================================== + +With Clang 12, the following bpf_verif_scale test failed: + * ``bpf_verif_scale/loop6.bpf.o`` + +The verifier output looks like + +.. code-block:: c + + R1 type=ctx expected=fp + The sequence of 8193 jumps is too complex. + +The reason is compiler generating the following code + +.. code-block:: c + + ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) { + 14: 16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6> + 15: bc 51 00 00 00 00 00 00 w1 = w5 + 16: 04 01 00 00 ff ff ff ff w1 += -1 + 17: 67 05 00 00 20 00 00 00 r5 <<= 32 + 18: 77 05 00 00 20 00 00 00 r5 >>= 32 + 19: a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4> + 20: b7 05 00 00 06 00 00 00 r5 = 6 + 00000000000000a8 <LBB0_4>: + 21: b7 02 00 00 00 00 00 00 r2 = 0 + 22: b7 01 00 00 00 00 00 00 r1 = 0 + ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) { + 23: 7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1 + 24: 7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5 + +Note that insn #15 has w1 = w5 and w1 is refined later but +r5(w5) is eventually saved on stack at insn #24 for later use. +This cause later verifier failure. The bug has been `fixed`__ in +Clang 13. + +__ https://reviews.llvm.org/D97479 + +BPF CO-RE-based tests and Clang version +======================================= + +A set of selftests use BPF target-specific built-ins, which might require +bleeding-edge Clang versions (Clang 12 nightly at this time). + +Few sub-tests of core_reloc test suit (part of test_progs test runner) require +the following built-ins, listed with corresponding Clang diffs introducing +them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too +old to support them, they shouldn't cause build failures or runtime test +failures: + +- __builtin_btf_type_id() [0_, 1_, 2_]; +- __builtin_preserve_type_info(), __builtin_preserve_enum_value() [3_, 4_]. + +.. _0: https://reviews.llvm.org/D74572 +.. _1: https://reviews.llvm.org/D74668 +.. _2: https://reviews.llvm.org/D85174 +.. _3: https://reviews.llvm.org/D83878 +.. _4: https://reviews.llvm.org/D83242 + +Floating-point tests and Clang version +====================================== + +Certain selftests, e.g. core_reloc, require support for the floating-point +types, which was introduced in `Clang 13`__. The older Clang versions will +either crash when compiling these tests, or generate an incorrect BTF. + +__ https://reviews.llvm.org/D83289 + +Kernel function call test and Clang version +=========================================== + +Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support +to generate extern function in BTF. It was introduced in `Clang 13`__. + +Without it, the error from compiling bpf selftests looks like: + +.. code-block:: console + + libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2 + +__ https://reviews.llvm.org/D93563 + +btf_tag test and Clang version +============================== + +The btf_tag selftest requires LLVM support to recognize the btf_decl_tag and +btf_type_tag attributes. They are introduced in `Clang 14` [0_, 1_]. +The subtests ``btf_type_tag_user_{mod1, mod2, vmlinux}`` also requires +pahole version ``1.23``. + +Without them, the btf_tag selftest will be skipped and you will observe: + +.. code-block:: console + + #<test_num> btf_tag:SKIP + +.. _0: https://reviews.llvm.org/D111588 +.. _1: https://reviews.llvm.org/D111199 + +Clang dependencies for static linking tests +=========================================== + +linked_vars, linked_maps, and linked_funcs tests depend on `Clang fix`__ to +generate valid BTF information for weak variables. Please make sure you use +Clang that contains the fix. + +__ https://reviews.llvm.org/D100362 + +Clang relocation changes +======================== + +Clang 13 patch `clang reloc patch`_ made some changes on relocations such +that existing relocation types are broken into more types and +each new type corresponds to only one way to resolve relocation. +See `kernel llvm reloc`_ for more explanation and some examples. +Using clang 13 to compile old libbpf which has static linker support, +there will be a compilation failure:: + + libbpf: ELF relo #0 in section #6 has unexpected type 2 in .../bpf_tcp_nogpl.bpf.o + +Here, ``type 2`` refers to new relocation type ``R_BPF_64_ABS64``. +To fix this issue, user newer libbpf. + +.. Links +.. _clang reloc patch: https://reviews.llvm.org/D102712 +.. _kernel llvm reloc: /Documentation/bpf/llvm_reloc.rst + +Clang dependencies for the u32 spill test (xdpwall) +=================================================== +The xdpwall selftest requires a change in `Clang 14`__. + +Without it, the xdpwall selftest will fail and the error message +from running test_progs will look like: + +.. code-block:: console + + test_xdpwall:FAIL:Does LLVM have https://reviews.llvm.org/D109073? unexpected error: -4007 + +__ https://reviews.llvm.org/D109073 |